The importance of pulmonary macrophages in the lung is increasingly evident. Not only do they ingest and kill pathogens but their secretory products regulate the activities of lymphocytes, fibroblasts, and other cells. Changes in the renewal of lung macrophages are related to environmental stimuli. We have identified an antigen specific for pulmonary macrophages and have evidence that this antigen develops only after these cells come onto the alveolar surface. The purpose of this research is to quantitate antigen on pulmonary macrophages and then to determine the distribution of cells with different quantities of antigen in populations of pulmonary macrophages obtained by lavage. We will determine the relationship between the development of this antigen and the age and extent of differentiation of these cells. Age will be determined in studies correlating presence of antigen and in vivo tritiated thymidine labeling. Differentiation will be determined on the basis of enzyme activity in macrophages and ability to phagocytose particles in vitro. Enzymes to be studied include acid phosphatase, beta-glucuronidase, cathepsin B and esterase. Fluorescence activated flow cytometry and sorting will be the primary method used for identification and quantitation of antigen and markers of differentiation. Once these relationships are established, we hope to use this antigen as a marker to detect and separate less differentiated populations of macrophages for in vitro study of factors which regulate differentiation and development. After defining populations of macrophages in normal animals by these in vitro and in vivo parameters, we will determine the distribution of cells with different quantities of antigen after in vivo instillation of inert, toxic and infectious agents. Determination of the differentiation and functional capabilities of macrophages within these populations should provide evidence that acquisition of the antigen is also a reliable indicator of activation induced by environmental and infectious agents.